1. Field of the Invention
This invention relates to a device for producing hydrogen for portable power applications. This invention further relates to a solar-powered device for producing hydrogen for portable power applications. This invention further relates to direct photoelectrolysis of water to generate hydrogen directly with sunlight employing a combination of advances in fuel cell technology, photovoltaic technology, photoelectrochemistry, and thin film technology. Finally, this invention relates to a photoelectrode employing a nano-crystalline catalyst material for use in the photoelectrolysis of water to generate hydrogen.
2. Description of Related Art
Hydrogen is an important future energy carrier and energy storage medium. Efficient, low-cost methods of making hydrogen will be important elements of a future hydrogen economy. Renewable electricity integration for hydrogen production is very important for reducing greenhouse gases and oil dependence of the U.S. A renewable hydrogen production device must be highly efficient, must have a long lifetime, and must be low in cost. Solar energy represents the best renewable energy source for such hydrogen production devices. Given the existence of an abundant supply of solar energy, solar photovoltaic, photoelectrochemical, or photocatalytic hydrogen generation can become viable technologies. However, to make this a reality, it is necessary to reduce costs, increase efficiency, and improve service life.
For present-day solar photovoltaic cell-driven electrolysis, the overall efficiency is the product of the efficiency of the solar cell and the efficiency of the electrolyzer. Solar cell efficiencies have been reported from 6% to as high as 32% depending upon the materials used. Conventional electrolyzer efficiency is approximately 75%. Thus, solar cell-driven electrolysis efficiency can be in the range of about 4.5% to about 24%; however, in practice, only values at the low end of this range are encountered. These low efficiencies are due in part to efficiency losses from sunlight absorption by a liquid electrolyte layer, impediments to the departure of product gases from the photoelectrodes due to electrolyte surface tension, and high overpotential of the photoelectrodes. In addition, system life is limited by photocorrosion and electrochemical corrosion of the electrode. Finally, in addition to all of the mechanical and operational issues, the costs of these devices are too high for wide use.
The current design of photoelectrodes is an additional hindrance to the development of improved photoelectrochemical systems because the semiconductors employed therein are fabricated on conductive substrates. With this type of design, there is no way to reduce the thickness of the electrolyte layer and eliminate the surface tension that acts as an inhibitor to the release of product gases because the reactant water and electrolyte must be transported to the front of the electrode.
Numerous efforts have been made to enhance the efficiency and stability of photoelectrochemical cells. The general approach has been to coat a layer of protective materials, which may be organic substances, active metal ions, noble metals, light sensitive dyes and more stable semiconductors, such as metal oxides, onto the photoelectrode surface. Recent developments include a thin film dye to sensitize the semiconductor electrodes in photoelectrochemical cells. Although the use of light sensitive dyes on the semiconductor electrode surface has improved the light absorption efficiency thereof, it is still necessary that the mass transport rate be increased and that the electrolyte thickness be reduced.
A 1 kw proton exchange membrane fuel cell stack (PEMFC) at 12 V requires about 84 A current. If the PEMFC is utilized 8 hours per day, power on the order of 8 kWhr/day is required. For power of this magnitude, assuming that hydrogen consumption in the fuel cell stack is 100%, the amount of hydrogen required to meet this demand is about 7.6 ml H2/A-min, i.e. 456 ml H2/A-hr. Thus, for a 1 kW fuel cell stack at 12 V, I=84, 666.7 A-hr/day, about 304.1 liters (666.5 A-hr/day×0.456 liters H2/A-hr) of hydrogen at room temperature and ambient pressure is required. This amount of hydrogen is equivalent to about 13.57 moles or 27 grams H2. However, in reality, hydrogen utilization by the fuel cell is only about 90%. Thus, 15 moles or 30 grams or 338 liters of H2 at room temperature and ambient pressure are required. For a hydrogen storage tank having a volume of 17.1 liters, hydrogen pressurized to 19.8 atmospheres is required.
Electrochemical proton exchange membrane hydrogen concentrators/compressors are known in the art (e.g. Giner, Inc.) and generally employ two reaction steps:Anode: H2(dilute)=2H++2e Cathode: 2H++2e=H2 In such devices, a hydrogen-containing gas stream at ambient pressure is fed to the anode compartment of the cell to produce protons by means of external electricity. The protons pass through the proton exchange membrane and generate hydrogen gas. With bias voltage, the desired elevated hydrogen pressure is generated. The current density, J, at any point in the electrochemical hydrogen compressor is given as follows:J=V/ρ−b/ρ ln(P0/P1)where V is the uniform cell potential drop across the cell, ρ is cell resistivity in Ohms·cm2, b is RT/2F, P0 is the outlet hydrogen pressure, and P1 is the dilute hydrogen pressure. If hydrogen is fed to the anode at a pressure of 2 atma,E=29.5T/298 log(P/Patm)mV At 80° C. and 200 atma pressure differential, E=70 mV. In reality, the compressor has an internal polarization resistance (iR drop), as a result of which the overall voltage thermodynamically is much higher than 70 mV. In practice, the compressor, with a cell operating in a liquid-water-flooded cathode configuration produces 6.90 MPa hydrogen pressure from 207 kPa with an active area at 214 cm2, a cell voltage of 320 mV at 40° C., and a current density of about 2000 mA/cm2. As previously indicated, to meet the requirements of a 1 kW PEMFC stack for 8 hours, 30 grams of hydrogen are required. Stored in a 17.1 liter tank, the pressure is 19.8 atm. The current density will be about 1493 mA/cm2, i.e. a power density of 0.48 W/cm2.
The amount of power needed to produce 30 grams of hydrogen is defined byQ=nFwhere Q is coulombs, n is moles of hydrogen, and F is the Faraday constant. Accordingly,Q=2×15 moles×96500 C/mole=2.88×106 C To produce 30 grams of hydrogen, the stack is operated for 6 hours per day and the current of the electrolyzer isI=Q/t=2.88×106 C/(5 hours×3600 s/hours)=160 A If the electrolyzer is operated at 1.6 V, the power of the electrolyzer will be at least 1.6 V×160 A=256 W. However, a commercial electrolyzer has an efficiency of 75%. Thus, the actual electrolyzer power must be 366 W.
A conventional solar panel has a power density of about 15 W/0.22 m2 or 68 W/m2. To produce 1 kW PEMFC operation for 8 hours requires a solar panel having a size of about 5.4 m2. The conversion of solar energy to hydrogen has a relatively low efficiency of about 4.9%.
Additional challenges associated with photoelectrochemical water splitting include materials efficiency, materials durability, bulk material synthesis, device configuration, and system design and evaluation. In the case of materials efficiency, materials with smaller bandgaps utilize the solar spectrum more efficiently, but are less energetically favorable for hydrogen production because of the mismatch with either the hydrogen or oxygen redox potentials. Thus, there is a need for materials with appropriate bandgap for hydrogen production. In the case of materials durability, the high-efficiency materials currently available corrode quickly during operation, and the most durable materials are very inefficient for hydrogen production. Thus, there also is a need for durable materials with the appropriate characteristics for photoelectrochemical hydrogen production.
Based upon a screening of semiconductor materials used in conventional photoelectrochemical conversion processes, the narrow bandgap materials, such as II-VI series (CdS, CdSe, CdTe), III-V (InP), CIS (CuInSe2), all have broad sunlight absorption spectra, but the stability of these materials is low. Metal semiconductors, by comparison, such as WO3 and TiO2, have large bandgaps and are highly stable, but the efficiencies resulting from the use of these materials are low.
Many attempts have been made to improve the metal oxide photo conversion efficiency. Recent reports have indicated that TiO2 and WO3 are very attractive for photo splitting of water for hydrogen production. Park, J. H. et al., Electrochemical and Solid-State Letters, 9(2) E5-E8, 2006 teaches an assembled photoelectrochemical tandem cell with bipolar dye-sensitized electrodes for water splitting for which a 2.5% solar-to-hydrogen efficiency was obtained with 0.2 V positive bias. Khan, S. U. M. et al., Science, Vol. 297, 2243-5 (2002) discloses a maximum photo-conversion efficiency of 8.35% using a chemically modified n-TiO2 at 0.3 V positive bias. The chemically modified n-TiO2 was synthesized by a direct flame pyrolysis of a Ti metal sheet. As is apparent from the teachings of these two references, a direct method for producing nano TiO2 shows better photo conversion efficiency under similar bias conditions.